Field of the Invention
The present invention relates to an active material that is used as a negative electrode material for a non-aqueous electrolyte secondary battery such as a lithium ion secondary battery, in particular, silicon particles very useful as a negative electrode active material, and a method for manufacturing the same. Further, the present invention relates to a negative electrode for a non-aqueous electrolyte secondary battery that uses the negative electrode material and a non-aqueous electrolyte secondary battery that uses the negative electrode material.
Description of the Related Art
With the remarkable development of portable electronic devices and communication devices in recent years, there is a strong demand for a non-aqueous electrolyte secondary battery having higher energy density from the viewpoint of economic efficiency and reduction in size and weight of devices.
Since silicon has a theoretical capacity of 4200 mAh/g far higher than a theoretical capacity of 372 mAh/g of a carbon material that is put in practical use at the present time, it is a material most expected in miniaturization and higher capacity of a battery.
Silicon is known in various forms different in a crystal structure according to a manufacturing method thereof. For example, patent document 1 discloses a lithium ion secondary battery that uses single crystal silicon as a support of a negative electrode active material. Patent document 2 discloses a lithium ion secondary battery that uses a lithium alloy LixSi (x: 0 to 5) of single crystal silicon, polycrystalline silicon and amorphous silicon, LixSi that uses amorphous silicon is particularly preferred, and pulverized crystalline silicon covered with amorphous silicon obtained by plasma decomposition of monosilane is illustrated. However, in this case, as shown in Example, 30 parts by weight of silicon and 55 parts by weight of graphite as a conductive agent were used; accordingly, battery capacity of silicon could not be fully exerted.
Further, patent documents 3 to 5 disclose a method where an amorphous silicon thin film is deposited on an electrode current collector by vapor deposition and this is used as a negative electrode. There is also disclosed a method where in the method where silicon is directly vapor grown on the current collector, when a direction of growth is controlled, the cycle characteristics can be prevented from deteriorating due to volume expansion (see patent document 6). According to this method, it is said that an electrode having high capacity and excellent cycle characteristics can be manufactured. However, there were problems that due to limited production speed, the cost is high, a silicon thin film is difficult to make thicker, and copper that is a negative electrode current collector diffuses into silicon.
Accordingly, in recent years, a method where while using silicon particles, a usage rate of silicon in battery capacity is restricted to suppress volume expansion (patent documents 7 to 9), a method where as a method where a grain boundary of polycrystalline particles is used as a buffering region of the volume change, a method where a silicon melt in which alumina is added is quenched (see patent document 10), polycrystalline particles configured of mixed phase polycrystals of α, β-FeSi2 (see patent document 11), and a high-temperature plastic forming process of a single crystal silicon ingot (see patent document 12) are disclosed.
As was described above, in order to use silicon as an active material, metallic silicon and silicon alloys, which have various kinds of crystal structures, have been proposed. However, all of these are disadvantageous from the viewpoint of the cost. That is, a method that allows to mass produce at low cost has not been proposed.